this converts existing pkg-config checks to use check_package, which
sets up all the needed variables after a check.
This also disables libdl and libiio for static builds, where they make
no sense
Before:
Checking for code snippet : yes
Checking for code snippet : yes
Checking for code snippet : yes
Checking for code snippet : yes
Checking for code snippet : no
Checking for code snippet : no
After:
Checking for HAVE_CMATH_ISFINITE : yes
Checking for HAVE_CMATH_ISINF : yes
Checking for HAVE_CMATH_ISNAN : yes
Checking for NEED_CMATH_ISFINITE_STD_NAMESPACE : yes
Checking for NEED_CMATH_ISINF_STD_NAMESPACE : no
Checking for NEED_CMATH_ISNAN_STD_NAMESPACE : no
The usual name for this header is config.h, but that's already used by
vehicles. Using uppercase could give the impression this is a
file to be modified, but it's not. Use lowercase instead.
